1. Field of the Invention
The invention relates to a process for operating and a calender with a roll stack that features two end rolls in a press plane with several middle rolls located between the end rolls. At least one of the rolls includes an elastic surface.
2. Discussion of Background Information
The invention is described below on the basis of a calender that is used for glazing paper or cardboard webs. However, it can also be used in the same way with other material webs with which similar problems occur.
When glazing a paper web, the paper web is guided through the calender and into nips that are formed between a hard and a soft roll, i.e., a roll with an elastic surface, and is acted upon by increased pressure and, if necessary, also by increased temperature. In the case of calenders of more recent construction types, e.g., the “Janus calenders,” rolls are used that are covered with a plastic coating. It can now be observed that in many cases crosswise stripes occur on the paper web after a certain working time. As soon as these crosswise stripes become visible, the paper web becomes useless and forms broke. The reasons for this so-called barring formation have not yet been conclusively established. However, it is assumed that they are the effects of a vibration phenomenon. However, vibrations are virtually unavoidable in a calender.
Barring phenomena per se have also occurred earlier, namely with calender stacks, i.e., calenders that were equipped exclusively with hard rolls. However, in this case, the reasons for the barring formation are assumed to lie in the paper web, i.e., the periodic occurrence of changes in thickness, which were caused, e.g., by a slightly pulsating headbox.
In the case of calender stacks, it has been tried to prevent such a barring formation either by arranging a guide roll at alternating distances from the roll stack, or by laterally displacing one or more rolls from the press plane.
However, in the case of barring formation on soft rolls, in particular plastic rolls, this is a different phenomenon. Here it can be observed that the elastic surface layer changes by itself within a relatively short time. When a barring phenomenon occurs, the roll that exhibits the barring formation must be removed and reground or finished. The service life of such a roll is therefore limited.
In the barring formation the soft roll is changed on its elastic surface. It has not yet been conclusively determined what this change actually entails. The following possibilities are currently assumed: the roll develops a waviness on the surface, i.e., a hill and valley structure, the roll becomes polygonal, or the roll develops alternating zones of varying surface quality in the circumferential direction, e.g., varying roughness. Regardless of the concrete type of change, after the barring formation periodic stripes appear on the circumference of the roll running in the axial direction. Corresponding stripes then appear on the paper web, whereby the paper web is to be considered as broke by the time the stripes become visible at the latest.